The present application relates to a battery and, more particularly, to a battery in which an open circuit voltage per pair of cathode and anode in a perfect charging state is equal to or larger than 4.25V.
In recent years, techniques for portable electronic apparatuses have remarkably been developed. Electronic apparatuses such as cellular phone, notebook-sized computer, and the like have been started to be recognized as fundamental technologies for supporting an advanced information society. Studies and developments regarding a technique for realizing advanced functions of those electronic apparatuses are vigorously being progressed. In proportion to it, electric power consumption of those electronic apparatuses is increasing more and more. On the contrary, since it is demanded that those electronic apparatuses can be driven for a long time, it is inevitably demanded to realize a high energy density of a secondary battery serving as a driving power source. It is also demanded to elongate a cycle life in consideration of an environmental view.
The higher energy density of the battery is preferable from viewpoints of an occupation volume, a mass, and the like of the battery built in the electronic apparatus. At present, since a lithium ion secondary battery has an excellent energy density, such batteries have been built in most of the apparatuses.
Ordinarily, in the lithium ion secondary battery, lithium cobalt acid is used for a cathode and a carbon material is used for an anode. An operating voltage is set to a value within a range from 4.2V to 2.5V. A terminal voltage in a cell can be raised up to 4.2V mainly owing to an excellent electrochemical stability of a non-aqueous electrolyte material, a separator, or the like.
In the lithium ion secondary battery in the related art which operates at the maximum voltage of 4.2V, a cathode active material such as lithium cobalt acid or the like which is used for the cathode uses a capacitance of at most about sixty percent of its theoretical capacitance. Therefore, a residual capacitance can be utilized in principle by further raising a charge voltage. It has actually been known that the high energy density can be realized by setting the voltage upon charging to 4.25V or more (for example, refer to Patent Document 1: International Publication No. WO03/019713).
However, in the battery whose charge voltage has been set to 4.25V or more, particularly, an oxidation atmosphere near the cathode surface is enhanced, so that an electrolytic solution which physically comes into contact with the cathode is oxidation decomposed and charge/discharge efficiency deteriorates. Further, when a reaction to the electrolytic solution on the anode side continues, conductivity of a coating deteriorates, metal lithium is precipitated on the anode due to an excessive doping of lithium, and cycle characteristics deteriorate.
Particularly, if the charge voltage is set to 4.25V or more, when the cycle is repeated, a capacitance maintaining ratio deteriorates suddenly and there is a case where it is difficult to predict the residual capacitance and sufficiently effect an advantage obtained by increasing the capacitance owing to the high charge voltage. Since it is difficult to sufficiently suppress such a sudden deterioration occurring after the elapse of predetermined cycles according to the electrolytic solution or the like constructing the battery in the related art, it is necessary to newly examine constructing materials such as an electrolytic solution and the like.